1. Field of the Invention
The present invention relates generally to spoked wheels and bicycles including such wheels. More specifically, the invention relates to tensioned spoked wheels which are low in weight, high in stability and especially suited for use on bicycles.
2. Description of the Prior Art
The art of tensioned spoked wheels is one which dates back well into the 1800's when such wheels were developed for the Highwheeler bicycles and Ordinaries of the 1880's. Prior to that time, compressively loaded spoked wheels were standard fare as evidenced by Roman chariot wheels, long ago, and, more recently, by the wheels of the Ford Model T automobile. An example of the compressively loaded spoked wheel in the context of a bicycle wheel is shown in U.S. Pat. No. 452,649 (Powell).
U.S. Pat. No. 339,550 (Hudson) discloses a tensioned spoked wheel assembly from the heyday of the Ordinaries. This patent is particularly concerned with the construction of a rim from tubing or sheet metal and includes a seam which is protected from the elements by being positioned under the tire. In cross section, this wheel assembly is illustrated as having a toroidal rim which is wider than a tire mounted on it.
U.S. Pat. No. 5,061,013 (Hed et al.) discloses a bicycle wheel with good aerodynamic properties. The wheel has a toroidal rim with a high aspect ratio and a width exceeding the width of a tire to be mounted on it. FIG. 4 of the patent illustrates a wheel with 14 spokes. This is a reduced spoke count wheel in the sense that modern mass-produced bicycle wheels typically have 32 to 48 spokes. This is a conventionally spoked wheel in the sense that the outer ends of the fourteen spokes are connected to the rim at 14 points which are evenly spaced about the circumference of the rim. The inner ends of the fourteen spokes are connected to the hub with seven spokes on each side of the hub. The fourteen spoke wheel illustrated in FIG. 4 of the patent is a conventional radial spoked wheel. It is worth noting that the United States Cycle Federation (USCF) enforces a sixteen spoke minimum, per wheel, for bicycles involved in sanctioned, mass-start races. This type of spoking will be referred to herein as conventional spoking.
U.S. Pat. No. 2,937,905 (Altonburger) discloses a rim configuration with a novel spoke connection in which the spoke nipple rests upon a surface which is canted so that spoke forces are well distributed on the nipple seat portion of the rim.
U.S. Pat. No. 4,583,787 (Michelotti) discloses a nipple seat bushing which is slanted to achieve a better stress distribution. U.S. Pat. No. 4,729,605 (Imao et al.) discloses a spoke with a fiber reinforced central portion and two fittings, one at each end of the central portion.
Tests have shown that each spoke of a modern quality bicycle wheel has an elastic limit, or yield point, of +300 kg in tension, approximately 4 times static tension. Since this yield point exceeds the total rider-machine weight by a factor of approximately 3 for an 175 lb rider and 25 lb bicycle, builders have become more daring in lowering spoke count. The minimum acceptable spoke count for mass-start United States Cycle Federation sanctioned races is 16 for a tensioned wheel. Conventional tensioned wheels with spoke counts below this have poor structural characteristics and become dangerously unstable and endanger not only the individual user but also other ride-race participants. Specifically, these low spoke count conventional wheels induce a steering input under load which becomes proportionately larger with each additional spoke count reduction and exhibit varying friction at the wheel-tire road contact point at a lean angle in turns. Measurements show that on a conventional fourteen spoke radially laced front wheel, the wheel axle departs from the horizontal, alternately dipping on the left side by net 0.015 inches when a right spoke passes over the wheel-road contact point (RCP) and then dipping net 0.015 inches on the right side when the next spoke, a left spoke, passers over the RCP. These horizontal position changes of the axle are measured with a 150 lb load applied at the axle at wheel center through the bicycle fork and are measured from axle center at the fork dropouts to the RCP on a 700C conventional fourteen spoke radially laced front bicycle wheel. Measurements show that the distance from the axle at the right dropout to the RCP decrease by 0.010 inches under load compared to the no-load distance as a right spoke is centered over the RCP and the distance from the axle at the left dropout to the RCP decreases by 0.025 inches under load compared to the no-load distance as the same right spoke is centered over the RCP. These differential distance variations result in a net 0.015 inch departure of the axle from the horizontal at the fork dropouts and this departure alternates from a low left dropout with the passage of a right spoke over the RCP to a low right dropout with the passage of the next, a left spoke, over the RCP. The rider experiences these horizontal axle position changes as alternating left to right and right to left steering inputs at the handlebar with the steering bar experiencing a direction reversal with the passage of each spoke over the RCP as the wheel rotates under load. The fourteen spoke wheel under discussion exhibits 14 such steering pulses per wheel revolution. With a higher spoke count conventional tensioned wheel these net axle departures from the horizontal become less and move to 0 for a solid wheel and these departures become more with a further reduced spoke count.
The amplitude and frequency of these steering vibrations are inversely proportional, the kinetic energy per cycle driving them being constant. As wheel rotation speed goes up, frequency goes up and amplitude goes down. As rotational speed goes down, amplitude goes up and frequency goes down. This tends to obscure the phenomenon to the inattentive rider. Energy is consumed by these vibrations, detracting from overall vehicle efficiency. As well, internal stresses are created in the wheel which eventually lead to system failure even if the wheel is run on a glass-smooth surface for its life cycle. In addition, these vibrations at the steering bar limit the lean angle a cyclist can achieve in a high-speed turn, where a constant steering angle is essential for safety once a lean angle has been established. These conventional wheel-induced steering inputs make a constant steering angle impossible. These steering inputs can also be the source of hitherto unexplained wheel shimmy on high spoke count conventional wheels when a highly tensioned spoke lies next to a low tensioned spoke as characteristically happens at the wheel-rim seam. Almost all wheels exhibit a variation in spoke tension at this point in the wheel and the net differential dip at the front dropouts will be much greater than 0.015 inches if adjoining spoke tension departs significantly. This greater steering pulse can at certain speeds, in concert with fork and frame characteristics, vehicle load distribution and rider-induced frame flex, cause sudden, uncontrollable and extremely dangerous shimmy during high vehicle speed.
At the rear of the bicycle the low spoke count conventional wheel cannot exhibit axle departure from the horizontal as the position of the dropouts is fixed in space by the closed triangles formed by the seat stays, chain stays and seat tube, the dropouts being attached at the intersection of the seat and chain stays. Axle movement being thus restricted, the geometry of the conventional wheel, specifically the spoke pattern, pulls the rim out of the center plane of the wheel at the RCP under load. The conventional fourteen spoke radially laced front wheel was tested under a 150 lb load applied at the axle with the axle locked in fixture restricting any axle movement as it would be were it installed at the rear of a bicycle and the departure from the wheel center plane of the rim was measured at the RCP. During this test the RCP was free to move and the axle was fixed. The wheel exhibited a lateral departure of 0.100 inches out of its center plane away from the spoke centered over the RCP. That is, when a right spoke was centered over the RCP the rim was deflected to the left and when a left spoke was centered over the RCP the rim was deflected to the right. The RCP would thus describe a sine wave over the road surface with an amplitude of 0.200 inches; 0.100 inches on each side of the wheel center plane as successive alternate spokes pass over the RCP with the distance between adjoining right peak side departures measured along the vehicle center line of travel being equal to the distance between adjoining right spokes projected to the RCP. These lateral side-to-side deflections of the rim at the RCP of a loaded moving rear conventional wheel cause excess stress in the wheel and lead to early system failure, even if the wheel is always ridden on glass-smooth surfaces. Also, energy is consumed by the forces deflecting the rim laterally and this again detracts from overall system efficiency. During high speed cornering these side-to-side deflections severely limit the lean angle because road contact friction is severely pulsed going from a minimum to a maximum and back with the passage of successive spokes over the RCP.
The differential up and down rocking of the front fork dropouts and the lateral rear rim deflection at the RCP in loaded dynamic conditions of conventionally spoked tensioned wheels are caused by the existence of a horizontal force gradient (considering the wheel center plane as vertically oriented) in the rim between the spoke-rim contact points. The force applied by each spoke at the rim can be resolved into horizontal and vertical components and a typical horizontal component is 23 lbs. Thus a left spoke tensioned to about 150 lbs (typical) pulls the rim to the left, out of the wheel center plane with a resolved force of about 23 lbs. The next spoke along the wheel rotation will be a right spoke and it pulls the rim to the fight by about 23 lbs if uniform tension exists. In an unloaded conventional wheel these forces are in balance and the rim is centered in the wheel center plane which lies halfway between the dropouts and a force gradient perpendicular to the plane of the wheel exists from spoke to spoke along the rim. On a low spoke count conventional wheel the distance along the rim between spokes becomes greater going from about 2 inches on a conventional thirty six spoke 700C rim to 5.25 inches on a conventional fourteen spoke wheel of the same diameter. Thus as any given spoke passes over the RCP on a low spoke count conventional wheel the adjoining spokes, the one directly ahead and behind, carry relatively less of the load and remain relatively high in tension and since each of these directly adjoining spokes is of opposite orientation of the main load carrying spoke and since this main load carrying spoke is severely reduced in tension, no countervailing or a severely reduced countervailing force vector remains and thus no alternating force vector remains to balance the horizontal force vectors of the adjoining spokes. If a right spoke is positioned over the RCP and it is substantially unloaded in tension by the system load its horizontal force vector along with its vertical force vector is essentially reduced to a very low value and the adjoining spokes being both of left orientation pull the rim unopposed to the left. As the conventional bicycle wheel rolls along under load the next spoke to become unloaded by the system load will be a left spoke and the rim will be deflected to the right resulting in a zigzag trace of the RCP along the wheel line of travel if a print were left by the RCP on the pavement. With further reduction in spoke count in a conventional wheel the amplitude of the zigzag trace will increase and with a greater spoke count the zigzag trace will decline in amplitude, going to 0 for a solid wheel.
In order to reduce the weight of tensioned spoked wheels, wheel makers have looked to low spoke count wheels such as the fourteen spoke wheel disclosed in Hed et al. Such a construction, however, suffers from instability and the origin and consequences of this instability are discussed herein in great detail. In Hed et al., it is suggested that one can produce a reduced spoke count wheel with as few as eight spokes if one uses a rim that is stiff and strong enough. As explained herein, the need for a stiff rim in low spoke count, conventionally laced wheels arises because of a practical limitation on the minimum number of spokes that one can use in making a conventional tensioned spoked wheel. Specifically, in such wheels, there are certain side loads that are unresolved by the spokes and are resolved only in the rim. Resolution of these loads in the rim creates a steering input in front wheels. Generally speaking, as the number of spokes per wheel is reduced, the magnitude of these forces increases until a point is reached at which conventional rims simply can't hold up. One answer, suggested in Hed et al., is to use a stronger rim but this almost necessarily involves additional mass, however, and the goal of a reduced weight wheel is subverted in the process.
There remains a need for a reduced spoke count wheel which does not require a super strong rim. There is also a need, particularly in the context of reduced spoke count wheels, for improved stability with respect to lateral loading.